This invention relates to a control device for controlling the power supply to an oscillating motor for driving a compressor unit of the resonant-piston type, more particularly to a circuit for supplying alternating current from an electric power source to the oscillating motor.
The invention also relates to a compressor unit of the resonant-piston type provided with such a control device. Such a control device and compressor unit are known from the U.S. Pat. No. 4,345,442.
In the known compressor unit the piston is driven with an oscillating motion relative to the cylinder by means of an oscillating motor constructed as a linear motor. The frequency of this oscillating motion is substantially constant and is substantially equal to the resonant frequency of the spring-mass system constituted by the compressor and the oscillating motor. In order to obtain this oscillation the oscillating motor is energized with an alternating current having a constant frequency substantially equal to this frequency. Since the resonant system is mainly driven in resonance the power required for reciprocating the piston is relatively low, which makes it possible to use a motor with a lower driving power and hence smaller energizing currents. This has the advantage that the dimensions of the oscillating motor can be small, which is favorable for cost reasons. This is of particular importance in compressor systems for mass-manufactured products such as, for example, refrigerators and freezers. A known property of such compressors is that the resonant frequency varies continually as a result of changes in ambient parameters, such as for example temperature. This partly eliminates said advantage because the required driving power and energizing current increase rapidly as the difference between the actual operating frequency and the optimum operating frequency, which corresponds substantially to the resonant frequency, increases.